Topographic anatomy of the nervous systems of the trematodes Azygia lucii and Phyllodistomum angulatum was studied by the histochemical method of cholinesterase detection. The cholinergic part of the nervous system of A. lucii is represented by the brain, the ventral, dorsal and lateral longitudinal cords, by rather long complementary anterior branch of the ventral cord and by very short anterior branch of the dorsal cord. All cords are connected with numerous commissures. In the anterior region of the body, between the brain and the ventral sucker there are distributed 9 ring commissures. Behind the ventral sucker there has been revealed up to 18 commissures, 10 of which are ring ones. The regularity of the distribution of commissures is disturbed only in the region of the ventral sucker. The nervous system of such type can be regarded as a regular frequent orthogon. In Ph. angulatum has been revealed another type of the nervous system--an uneven orthogon. It is represented by the brain, 3 pairs of longitudinal nerve cords, small complementary anterior branches of ventral and dorsal cords, 5 ring commissure between the oral--and ventral suckers and one ventral commissure behind the ventral sucker. The origin of regular frequent orthogon in A. lucii and the irregular orthogon in Ph. angulatum is explained by certain morphological peculiarities of their structure; the body shape, musculature development and the reprodactive system localization. The morphological diversity being a characteristic for the trematode nervous system allows to relate them with Neorhabdocoela suborder Typhloplanoida.